code: def test_loss(self):
      head = PanopticFPNHead(
          num_things_classes=2,
          num_stuff_classes=2,
          in_channels=32,
          inner_channels=32,
          start_level=0,
          end_level=1,
      )
      x = [torch.rand((2, 32, 8, 8)), torch.rand((2, 32, 4, 4))]
      data_sample1 = DetDataSample()
      data_sample1.gt_sem_seg = PixelData(
          sem_seg=torch.randint(0, 4, (1, 7, 8))
      )
      data_sample2 = DetDataSample()
      data_sample2.gt_sem_seg = PixelData(
          sem_seg=torch.randint(0, 4, (1, 7, 8))
      )
      batch_data_samples = [data_sample1, data_sample2]
      results = head.loss(x, batch_data_samples)
      self.assertIsInstance(results, dict)
  
file_path: mmdetection/tests/test_models/test_seg_heads/test_panoptic_fpn_head.py
imports: import unittest
  import torch
  from mmengine.structures import PixelData
  from mmengine.testing import assert_allclose
  from mmdet.models.seg_heads import PanopticFPNHead
  from mmdet.structures import DetDataSample
label_desc: Use image node_id 2 for calling the TestPanopticFPNHead obj's underlying member method code with example usage: obj.test_loss() without return types